Known digital demodulation approaches typically use a power-of-2 integer number of samples-per-symbol (i.e. 1, 2, 4, 8 or more) to aid timing recovery. Multiple samples-per-symbol are used since the timing recovery is thus simplified. However, this known technique places a high performance requirement on a receiver's Analog to Digital Converter (ADC) sample rate in order to capture the incoming signal. Further, in known systems the “information” embodied in the samples concerning the incoming waveform is not fully utilized as the samples generally are not used to reconstruct the incoming waveform. Rather, the samples in the known prior art systems are only used to determine the optimal decision time and the corresponding symbol value at the decision time.
These known prior art approaches cause two main issues to become relevant for high data rate links: the spectrum of high symbol rate signals can greatly exceed the analog input bandwidth of “available” ADCs; and the ADC sample rate of “available” ADCs is insufficient to capture the number of samples per symbol needed to support accurate timing recovery. Wherein the term “available” means meeting the system or symbol rate requirements.
ADC sample rates limit the number of symbols-per-second that may be transferred and the ADC input bandwidth requires a certain spectral containment of the incoming communication signal. Prior art ADCs are limited by sample rates that limit the number of channel symbols-per-second that can be converted from an analog signal to a digital signal, and input bandwidth which necessitates a small spectral containment or frequency band for the communication signal. These two limitations inherent in known prior art systems are fundamental limits not only for high data rate communication systems, but also for low data rate communication systems where power or cost drivers may limit the choice of “available” ADC candidates. The problems of the known art described above may be put in context with the following numerical example. A data link requiring 32 Gbits/sec using a prior art system applying 16-Quadrature Amplitude Modulation (QAM) yields a symbol rate of 8 Gsym/sec which implies the ADC must provide 16 Gsamples/sec (at 2 samples per symbol) or more for timing recovery. The ADC analog bandwidth (at 0.25 dB) required is 5.4 GHz for a modest roll-off factor of 0.35. This performance is clearly outside the boundary of known ADCs available today or in the near-future. A typical approach to reduce the required input bandwidth and sample rate for ADCs in current communication systems is to employ known higher-order linear modulations (i.e. M-ary QAM to which pulse shaping may be applied or shaped M-ary phase shift keying (PSK) with M>2). For a given data rate, higher-order modulations improve the spectral efficiency of an information bearing signal and lower the channel symbol rate. This reduces the requirements on a receiver's ADC input bandwidth and sample rate. However, high-order linear modulations typically they have non-constant envelopes either by constellation design (i.e. QAM) or intentional channel filtering (i.e. the Nyquist pulse shaping used to control intersymbol interference for linear modulations). Non-constant envelope signals have a peak-to-average signal power-ratio (PAR) greater than unity, which in turn negatively impacts receiver dynamic range requirements. But perhaps the more deleterious effect is the waveform distortion encountered when the communication channel is nonlinear. Nonlinear channels are typically encountered where high-efficiency transmit power amplifiers are employed (i.e. satellite downlinks, wireless handsets). In most cases typical linear equalization approaches cannot be used to correct the distortion, and system performance can become unacceptable. While nonlinear equalizers are possible, they typically are unattractive for many applications from cost, complexity, and throughput considerations.
Known prior art communication systems providing Low Probability of Intercept (LPI) on a data stream typically rely on spread spectrum modulation techniques such as direct sequence or frequency hopping. These known approaches introduce complicated circuitry and reduce spectral efficiency. Additionally, alternatives are desirable for providing LPI on a data stream without relying on prior art solutions of spread spectrum.
In embodiments of the present invention, digital data is encoded into a waveform of arbitrarily narrow bandwidth so that the waveform may be sampled at a relatively low rate (less than or equal to 1 sample per symbol), and then reconstructed to an arbitrary number of samples-per-symbol in the receiver. The symbol duration is greater than the duration between successive symbols, thus creating Intersymbol Interference (ISI). It is through the deconstruction of the ISI that the encoded information is obtained.
The disclosed embodiments of the present invention do not rely on higher-order modulation formats of the prior art. Hence, the embodiments avoid the pitfalls of the prior art schemes when used with non-linear and/or power-limited and/or spectrally-limited channels. The embodied processes exploit the limited spectral extent of low h (modulation index) and low BTP (bandwidth-time product) of known binary Gaussian Partial Response Continuous Phase Modulation (PRCPM) waveforms. As is known in the art, the PRCPM waveform is a constant envelope partial response signaling format. Embodiments of the waveform have the same analytic form as Gaussian Minimum Shift Keying (GMSK), but use an entirely different regime of operating conditions.
For example, using h=⅛ and BTP= 1/32, a binary alphabet (M=2) and a “Gaussian” frequency pulse, embodiments of the present invention may attain 10 bps/Hz spectral efficiency (FIG. 1). This means that for a 32 Gbps link, the PRCPM waveform requires only 3.2 GHz of bandwidth, using the 95% spectral containment definition, and can be Nyquist sampled at rate of 6.4 Gsamples/second, which is well below the symbol rate (binary symbols) of 32 GsymboVsec. The ADC sample rate performance and bandwidth is also less demanding than the 16-QAM approach described above. By using lower modulation indices and lower time-bandwidth products to increase the spectral efficiency, the advantage of the PRCPM scheme over known higher-order linear modulations may be further increased.
In principle, any ADC may be used as an acquisition device with an appropriate selection of PRCPM waveform parameters. However, a trade-off exists between the demodulator complexity and latency as the spectral efficiency of the PRCPM waveform is increased. Therefore, in practice, the spectral efficiency and ADC performance requirements (analog bandwidth and sample rate) must be optimized for a particular system.
The prior art higher-order linear modulation waveforms, such as M-ary QAM with M=64,128,256 or more, could be used to reduce the symbol rate and provide a more compact spectrum. However, this option requires more transmit power than lower-order versions of the same modulation type (i.e. 16-QAM) to achieve the same Bit Error Rate (BER) performance. A similar comment applies to other linear modulation classes such as PSK. In contrast, the binary Gaussian PRCPM waveforms, embodied in the present invention has a significant power advantage over known high-order linear modulation waveforms. Additionally, if the channel is nonlinear, the potentially high peak-to-average-ratio (PAR) of linear modulations requires special considerations in the link design to mitigate the effects of the PAR such as for shaped/unshaped QAM and shaped PSK, and the decreased spacing of constellation points such as for shaped/unshaped PSK, which are not required by the present invention. These prior art mitigation approaches lead to decreased system performance in certain aspects of the communication system (i.e. bandwidth, throughput, power). No such mitigation and associated deleterious effects are necessary with the waveforms used in embodiments of the present invention.
Embodiments of the invention are useful for nonlinear channels found in, for example mobile handsets, portable power-sensitive communication devices, SATCOM downlink channels and other links where power and spectrum may be at a premium, both with respect to availability and cost. It is to be understood that the invention is not limited to the above examples. With a binary non-linear modulation format used in embodiments of the present invention, data rates are enabled far beyond what binary linear modulation formats are capable of, thereby potentially greatly increasing revenue in spectrally limited applications. Also comparing the data rates of known prior art higher-order linear modulations to the present inventive embodiments introduced here it is noted, while the data rates can be made comparable, the present inventive embodiments do not introduce the design difficulties encountered with linear modulations. The overall process used with embodiments of the present invention is compatible with software-defined radio architectures. Additionally, the disclosed embodiments have an inherent LPI feature not found in known prior art linear modulation formats.
The benefits of the present invention are manifold. The invention allows use of ADCs with limited input bandwidth and sample rates. The invention makes possible the use of very low sample rate ADCs, relative to the communication signal symbol rate, in a communication receiver without resorting to high-order modulation. The invention lowers the required sample rate of the receiver ADC which enables ADCs with greater resolution and less power to be used than might otherwise be possible. This advantage is not limited to high data rate applications. Furthermore, the waveform used in embodiments of the invention are useful where LPI is desirable due to the inherent ISI present in the received waveform. As the peak-to-average (PAR) signal power ratio of waveforms generated by use of the invention approaches unity, use in non-linear channels is facilitated.
Therefore, it is an object of the present invention to obviate the problems of the prior art by presenting a novel system for processing a communication signal comprising a continuous phase modulated waveform containing information symbols of an M-ary alphabet, i.e., M=2, 4, 8, 16, etc., an analog-to-digital converter (ADC) operating at a sample rate at or less than the symbol rate of the communication signal, an interpolator for digitally reconstructing the waveform and a demodulator for extracting the information symbols from the reconstructed waveform.
It is another object of the present invention to provide a novel method for communicating information symbols with spectral efficiency. An embodiment of the method includes selecting a pulse shape for a partial response continuous phase modulated (PRCPM) waveform, encoding the information into the waveform to form a communication signal and, transmitting the communication signal. The embodiment also includes receiving the communication signal, sampling the communication signal at a sample rate less than the symbol rate to obtain signal samples, reconstructing the waveform from the signal samples and extracting the information encoded into the waveform.
It is still another object of the present invention to provide a novel improvement for a method of demodulating a PRCPM waveform containing information symbols at a symbol rate Rs, wherein the waveform is converted from analog to digital in an ADC by sampling the waveform at a rate less than or equal to the symbol rate Rs.
It is yet another object of the present invention to provide a novel improvement for a communication system for transmitting and receiving information symbols via a PRCPM waveform at a high symbol rate by using a PRCPM waveform described by the equation:
      s    ⁡          (      t      )        =                              2          ⁢                      E            s                                    T          s                      ⁢          cos      ⁡              (                              2            ⁢            π            ⁢                                                  ⁢                          f              o                        ⁢            t                    +                      Φ            ⁡                          (                              t                ,                α                            )                                +                      ϕ            0                          )            
where Φ(t,α) is a continuous function, expressed as:
            Φ      ⁡              (                  t          ,          α                )              =          2      ⁢      π      ⁢                          ⁢      h      ⁢                        ∑                      k            =                          -              ∞                                ∞                ⁢                                  ⁢                              α            k                    ⁢                      q            ⁡                          (                              t                -                                  k                  ⁢                                                                          ⁢                                      T                    s                                                              )                                            ⁢        
where h is the modulation index, q(t) is the phase pulse shape, and Ts is the symbol duration and αk are information symbols. The phase pulse shape is related to the frequency pulse shape g(t) as:
      q    ⁡          (      t      )        =            ∫              -        ∞            t        ⁢                  g        ⁡                  (          τ          )                    ⁢                          ⁢              ⅆ        τ            
It is a further object of the present invention to provide a-novel method of secure communication between a transmitter and a target receiver in an environment of extraneous receivers. One embodiment of the method comprises encoding information symbols in a PRCPM waveform and transmitting from the transmitter the waveform at a symbol rate greater than the maximum sample rate of the extraneous receivers. Another embodiment uses the PRCPM approach which provides security by a unique low-probability of intercept (LPI) feature. This feature may be obtained by partial response signaling using low h and low BTP. The PRCPM approach to LPI does not require the bandwidth expansion of standard LPI techniques such as chip rate withering or fast frequency hopping. Another embodiment of the method comprises encoding information symbols in a PRCPM waveform and transmitting from the transmitter the waveform at a symbol rate greater than the maximum sample rate of the extraneous receivers. The method also includes receiving the waveform at the target receiver, sampling the waveform to obtain signal samples and reconstructing the waveform from the signal samples. The method may additionally include the step of demodulating the signal to yield the information bits of the transmitted message.
It is an additional object of the present invention to provide, in a communication system with a base station and at least one remote station, a novel method for transmitting a communication signal having a waveform with information symbols from the base station to the at least one remote station. An embodiment of the novel method comprises the steps of: providing to the base station the maximum conversion rate and spectrum limitation of one or more of the at least one remote stations and selecting, at the base station, phase pulse shape parameters (i.e. modulation index, phase pulse, BTP, duration) for the waveform based on the maximum conversion rate and spectrum limitations provided to the base station. The method also includes the step of transmitting from the base station the communication signal with the selected pulse shape parameters at a symbol rate greater than the maximum conversion rate of one or more of the at least one remote stations.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal or the claims, the appended drawings, and the following detailed description of the preferred embodiments